(a) Field of the Invention:
The present invention relates to an adapter lens system for use with photographic cameras which are utilized for photographing images formed by optical instruments for observation directly by man's eyes such as endoscopes, microscopes and telescopes.
(b) Discription of the prior art:
FIG. 1 shows a sectional view illustrating composition of an optical system of a telescope consisting of an objective lens 1 and an eyespiece 2. As is seen from this drawing, the optical system is so designed as to permit man's eye to observe an image formed at a nearly infinite distance by the eyepiece 2.
As a method for photographing an image formed by such a telescope, there has conventionally been known a means to shift the eyepiece 2 away from the objective lens 1 as shown in FIG. 2 (in the direction indicated by an arrow) so that a real image formed by the objective lens 1 is refocused on a film 4 by the eyepiece. However, this method makes correction of aberrations rather difficult since it uses the eyepiece 2 at different imaging magnifications levels for observation by man's eye and photographing an image. Speaking concretely, aberrations are aggravated in the optical system when it is set either for observation by man's eye or photographing with a camera.
As a means for correcting this defect, there has been known another method to employ an adapter lens as shown in FIG. 3. Speaking concretely, this method use a technique to arrange an adapter lens 3 on the rear side of the position of the eyepiece 2 for observation by man's eye so that the image formed at nearly infinite distance by the eyepiece 2 is refocused on the film 4. This method mades it possibly to permit photographing with favorably corrected aberrations so long as aberrations of the eyepiece 2 are corrected favorably for observation by man's eye.
A lens system used for this purpose (adapter lens system) may have a large F number for magnifying an image at a high ratio and a relatively simple composition. For adding an adapter lens system on the rear side of an eyepiece of an ocular observation instrument, however, a stop 5 of the adapter lens system 3 becomes the exit pupil of the eyepiece 2 and is located on the front side of the adapter lens system 3. When an image is photographed by using a body of a commercially availably single-reflex lens type of camera, on the other hand, it is necessary to prolong the back focal length of the adapter lens system 3 to a certain degree as is understood from FIG. 4.
The back focal length must be prolonged also when a half prism (a prism comprising a semi-transparent surface) for automatic exposure control is arranged on the rear side of the adapter lens system 3 as shown in FIG. 5. However, a large total length of the adapter lens system or a camera body will be inconvenient for photographing operations. Therefore, it is desired for the adapter lens system to have a total length as short as possible.